KR20200002842A - Manufacturing method of austenitic stainless steel slabs - Google Patents

Abstract

[PROBLEMS] To provide a continuous casting technique that stably and remarkably suppresses surface defects occurring in the longitudinal direction (casting direction) of an austenitic stainless steel continuous casting slab.
[Solution] In continuous casting of austenitic stainless steel, electric power is generated so that the long side flow in the reverse direction is generated on the molten steel in the depth region where the solidification shell thickness at the center of the long side direction is 5 to 10 mm. A method for producing an austenitic stainless steel slab in which casting conditions are controlled by applying electron stirring (EMS) to control casting conditions to satisfy 10 <ΔT <50 × F _EMS +10 ,. However, ΔT is the difference between the average molten steel temperature (° C.) and the solidification start temperature (° C.) of the molten steel, and F _EMS is a stirring strength index expressed as a function of the molten steel flow rate in the long side direction by electromagnetic stirring and the casting speed.

Description

Manufacturing method of austenitic stainless steel slabs

The present invention relates to a method for producing austenitic stainless steel slabs by continuous casting using electromagnetic stirring (EMS).

As a solvent method of austenitic stainless steels including SUS304, a continuous casting method is widely used. The obtained continuous casting slab can be set as a thin steel strip through the process of hot rolling and cold rolling. These days, the manufacturing technology is established, and the sheet steel of austenitic stainless steel is used as a product material for many uses. However, even in such a thin steel strip of austenitic stainless steel, surface scratches which are thought to be derived from surface defects of the cast slab may be present. By introducing a process of grinding the slab surface with a grinder, the problem of surface scratches in the thin steel strip is often avoided. However, the surface grinding by the grinder increases the cost. There is a demand for a technique for producing a continuous cast slab in which surface scratches in a thin steel strip are not a problem even if surface grinding is omitted.

Patent Literature 1 discloses a technique for reducing surface defects caused by oscillation marks in a continuous casting slab of austenitic stainless steel. Moreover, in continuous casting of steel, electromagnetic stirring (EMS; Electro-Magnetic Stirrer) is effective and widely used as a measure of suppressing foreign material mixing to a solidification shell (for example, patent document 2 etc.). Patent Literature 3 discloses an example in which bubble defects and cracks generated in a continuous cast slab of medium carbon steel or low carbon steel are reduced by performing electron agitation and setting the discharge angle from the immersion nozzle upward 5 °. However, even if these conventional techniques are applied to the austenitic stainless steel, it is difficult to stably and remarkably reduce the occurrence of surface scratches caused by the cast slab in the thin steel strip.

Patent Document 1: Japanese Patent Application Laid-Open No. 6-190507 Patent Document 2: Japanese Patent Application Laid-Open No. 2004-98082 Patent Document 3: Japanese Patent Application Laid-Open No. 10-166120 Patent Document 4: Japanese Unexamined Patent Publication No. 2005-297001 Patent Document 5: Japanese Unexamined Patent Application Publication No. 2017-24078

According to the inventors' studies, surface scratches, which are presently applied to thin steel strips of austenitic stainless steel, are particularly susceptible to applications requiring beautiful surface appearance, are mainly caused by cracks occurring in the longitudinal direction (ie, casting direction) of continuous casting slabs. It is confirmed that it is due to the accompanying surface defect. Hereinafter, the defect of the surface of this kind of slab is called "casting direction surface defect." The occurrence of surface scratches in the thin steel strip caused by the casting direction surface defects does not lead to a solution even when the oscillation marks as disclosed in Patent Document 1 are smoothed.

According to the inventors' investigation, it is thought that the casting direction surface defect of the said continuous casting slab arises as follows.

If the cooling in the mold of the continuous casting process becomes uneven, non-uniformity in the thickness of the solidification shell occurs, and after that, stresses caused by solidification shrinkage or molten steel static pressure are concentrated here, resulting in minute cracking. do. This appears as casting direction surface defects on the slab surface. Since the cracks do not grow to the depth enough to break the solidification shells that have already been formed, they do not lead to serious situations that interfere with continuous casting operations.

Although the cause of the above-mentioned local cooling rate decrease cannot necessarily be identified, when the location of the casting direction surface defects is observed, it is often dented than the surroundings, so that the solidification shell falls locally from the mold at the beginning of solidification. It is thought that a phenomenon occurs. In this, a plurality of factors, such as nonuniformity of inflow of mold powder and nonuniformity of deformation | transformation by solidification shrinkage of a solidification shell, can be considered. In addition, this kind of casting direction surface defects tend to be particularly problematic in austenitic stainless steel grades, compared with ferritic stainless steel grades, etc., but this is considered to be due to the difference in the solidification mode.

It is known that the nonuniformity of the cooling in the mold is encouraged under strong cooling conditions, and a method of suppressing the occurrence of surface defects in the casting direction of the slab surface has been conventionally proposed by slow cooling in the mold. For example, in Patent Document 4, it is proposed to increase the thermal resistance of the mold powder layer and to completely cool the solidification shell by using a mold powder that is easy to crystallize. However, the mold powder alone cannot be said to have sufficient effect of slow cooling, and the surface casting direction surface defects of the austenitic stainless steel slab have not been eliminated. In addition, the change of the mold powder is not easy because there is an influence on other quality factors such as oscillation mark depth or breakout occurrence. In Patent Literature 5, the cooling of the mold is achieved by filling a metal wall of the mold with a low thermal conductivity. However, this alone cannot completely suppress the casting direction surface defects of the slab surface. In addition, when this type of mold is applied, it cannot be applied only to the steel grades in which the casting direction surface defects are a problem, but is applied to all steel grades, and thus may cause other surface quality deterioration factors in such steel grades.

The present invention discloses a continuous casting technique that stably suppresses the above-mentioned "casting surface defect" occurring in the longitudinal direction (that is, the casting direction) of the continuous casting slab in austenitic stainless steel, An object of the present invention is to provide a continuous cast slab of austenitic stainless steel, in which surface scratches are hardly generated when processed to a thin steel strip even if the surface of the continuous cast slab is omitted.

In view of the above circumstances, the inventors have diligently studied a method of suppressing the casting direction surface defects on the surface of the austenitic stainless steel slab. As a result, a combination of "lower temperature of casting temperature" and "electron stirring in the mold" is used. We found a way to achieve uniform slow cooling. By applying this method, it was confirmed that it is possible to stably and stably suppress casting direction surface defects in existing continuous casting facilities. This invention is based on this knowledge.

That is, this invention discloses the following invention.

In continuous casting of steel using a mold whose contour shape of the mold inner surface cut into the horizontal plane is a rectangular shape, the two mold inner wall surfaces constituting the long side of the rectangle and the two mold inner wall surfaces constituting the short side are When the "short side" and the horizontal direction parallel to the long side are called the "long side direction" and the horizontal direction parallel to the short side is called the "short side direction",

From the immersion nozzle having two discharge holes provided in the center of the long side direction and short side direction in the mold, in mass%, C: 0.005 to 0.150%, Si: 0.10 to 3.00%, Mn: 0.10 to 6.50%, Ni: 1.50 to 22.00%, Cr: 15.00 to 26.00%, Mo: 0 to 3.50%, Cu: 0 to 3.50%, N: 0.005 to 0.250%, Nb: 0 to 0.80%, Ti: 0 to 0.80%, V: 0 to 1.00 %, Zr: 0 to 0.80%, Al: 0 to 1.500%, B: 0 to 0.010%, total of rare earth elements and Ca: 0 to 0.060%, remainder Fe and inevitable impurities, consisting of the following formula (4) The molten steel of the austenitic stainless steel of the chemical composition whose A value is defined as 20.0 or less is discharged, and at least in the molten steel near the solidification shell in the depth region where the solidification shell thickness at the central position of the long side direction is 5 to 10 mm. On the long side, an electric agitation (EMS) is performed by applying electric power to generate a long side flow in the opposite direction to each other, and the following Equation (1) is satisfied. A method for producing an austenitic stainless steel slab, in which continuous casting conditions are controlled.

10 <ΔT <50 × F _EMS +10... (One)

However, ΔT and F _EMS are represented by the following formulas (2) and (3), respectively.

ΔT = T _L -T _S ... (2)

F _EMS = V _EMS x (0.18 x V _C +0.71). (3)

Here, T _L is the average molten steel temperature (° C) at an average hot water depth of 20 mm at the 1/4 position in the long side direction and 1/2 position in the short side direction, and T _S is the solidification start temperature (° C) of the molten steel. , F _EMS is the stirring strength index, V _EMS is the long side average molten steel flow velocity (m / s) in the depth region where the solidification shell thickness at the center of the long side direction given by the electronic stirring is 5 to 10 mm, and V _C is the casting slab. It is a casting speed (m / min) corresponding to the advancing speed of a longitudinal direction.

A = 3.647 (Cr + Mo + 1.5 Si + 0.5 Nb)-2.603 (Ni + 30 C + 30 N + 0.5 Mn)-32.377. (4)

Here, the value of content of the said element represented by mass% is substituted at the place of the element symbol of Formula (4).

In the continuous casting, it is more preferable to control the continuous casting conditions so that the following Equation (5) is also satisfied. Instead of formula (5), the following formula (6) may be employed.

ΔT ≦ 25... (5)

ΔT ≦ 20... (6)

Moreover, it is more preferable to control continuous casting conditions so that Formula (7) below may also be satisfied. Instead of the formula (7), the following formula (8) may be employed.

F _EMS ? (7)

F _EMS ? (8)

The molten steel of the molten steel in the mold is shaken by the melt flow or vibration during continuous casting operation. "Average surface depth" is the depth of the vertical downward direction based on the average position of the surface of molten steel. "Long side direction 1/4 position and short side direction 1/2 position" are two places which sandwiched the center immersion nozzle in the mold. Average molten steel temperature T _L (degreeC) is an average value of the molten steel temperature in 20 mm of average water surface depths in each of these two places. The solidification start temperature T _S (° C.) is a temperature corresponding to the liquidus temperature.

According to the manufacturing method of the continuous casting slab of the present invention, in the continuous casting slab of austenitic stainless steel, the generation of the "casting surface defect" is remarkably suppressed, and the surface treatment of the continuous casting slab by the grinder is omitted. In the manufacturing process, it becomes possible to avoid the problem of surface scratches caused by the slab which appears in the thin steel strip of the austenitic stainless steel.

1 is a surface appearance photograph of an austenitic stainless steel continuous casting slab having a casting direction surface defect.
2 is a surface appearance photograph of an austenitic stainless steel cold rolled steel sheet in which surface scratches caused by casting direction surface defects of the slab have occurred.
3 is a cross-sectional structure photograph of the vicinity of the surface of an austenitic stainless steel continuous casting slab in which casting direction surface defects have occurred.
4 is a diagram schematically illustrating a cross-sectional structure cut into a horizontal plane at the height of the molten steel surface of the molten steel in the mold with respect to the continuous casting device applicable to the present invention.
FIG. 5 is a diagram showing symbols "P ₁ and P ₂ " of the long side direction 1/4 position and the short side direction 1/2 position in the mold shown in FIG. 4; FIG.
6 is a metal structure photograph of a cross section perpendicular to the casting direction of the austenitic stainless steel continuous casting slab according to the present invention obtained by a method using electronic stirring.
7 is a metal structure photograph of a cross section perpendicular to the casting direction of an austenitic stainless steel continuous casting slab obtained by a method without using electronic stirring.
8 is a graph plotting the relationship between ΔT and F _EMS .

In continuous casting, in general, a flux layer in which mold powder is melted is formed on the molten steel of the molten steel in the mold. The flux enters the gap between the molten steel and the mold from the hot water surface during casting, and forms a flux film between the solidification shell and the mold to perform lubrication of both. Usually, at the same casting direction position (the same depth from the hot water surface), the distance between the solidification shell and the mold separated by the flux film is almost equal, and the heat removal from the mold is almost equal. Become. However, there may be a case where the distance between the shell and the mold in the initial solidification becomes larger than the surroundings due to some cause such as foreign matter entering between the solidification shell and the mold. At this point, the surface of the solidification shell is dented more than the surroundings, and the cooling rate is lower than the surroundings. Therefore, solidification proceeds in a state where the thickness of the solidification shell is thinner than the surroundings. Viewed from the top in the casting direction, the state where the thickness of the solidified shell is thinner than the surroundings continues for a while until the influence of the factor (such as foreign matter mixing) that increases the distance is eliminated. . That is, the solidification shell inside the mold is formed with an area in which a thin portion of the solidification shell extends in the casting direction. Stress is concentrated in the thinned portion of the solidified shell, and if the surface layer portion is unable to withstand the stress, surface cracks that extend in the casting direction occur inside the mold. However, the crack is minute and does not lead to an accident (breakout) in which the molten metal leaks from there. It is thought that the "casting direction surface defect" produced in the continuous casting slab of the austenitic stainless steel is caused by such a mechanism.

The main austenitic stainless steels often solidify the δ ferrite phase as the primary crystal, but depending on the chemical composition, the production rate of the δ ferrite phase may be extremely low or the austenitic single phase solidification may be performed. . P and S, which are impurities in the steel, are more likely to be dissolved in the δ ferrite phase than in the austenite phase. Therefore, especially in steel grades in which the generation ratio of the δ ferrite phase is low, P, S, and the like segregate at the grain boundaries of the austenitic phase. It is easy to do it, and the intensity | strength of the location is reduced. Therefore, in the austenitic stainless steel, it is thought that the above-mentioned "casting direction surface defect" accompanying surface cracking is easy to produce | generate compared with ferritic stainless steel.

Casting defect surface defects accompanying the surface cracks are often observed in the length direction of the slab in the length of several centimeters to tens of centimeters. When the degree of surface cracking is very large by visual inspection of the slab, the work of intensively trimming the part with a grinder may be performed. However, surface cracks of this kind exist in shallow areas of the slab, and therefore usually do not develop into additional cracks by hot rolling or cold rolling. Therefore, especially in general-purpose steel grades, such as SUS304, it is common to advance to the process of hot rolling and cold rolling, without giving special surface treatment to a continuous casting slab. Casting scale surface defects of some magnitude present on the surface of the continuous casting slab appear as surface scratches continuously or intermittently extending in the rolling direction in the cold rolled steel sheet. Therefore, in order to obtain a high quality austenitic stainless steel cold rolled steel sheet, it is effective to produce as few slabs as possible in the direction of casting direction surface defects at the stage of continuous casting.

The surface appearance photograph of the austenitic stainless steel continuous casting slab which the large casting direction surface defect generate | occur | produced in FIG. 1 is illustrated. With respect to the long side of a photograph, the parallel direction corresponds to the longitudinal direction (casting direction) of the slab, and the perpendicular direction corresponds to the width direction of the slab. At the location of the arrow, casting direction surface defects exceeding 27 cm in length are seen.

The surface appearance photograph of the austenitic stainless steel cold rolled sheet steel in which the surface flaw resulting from the casting direction surface defect of the slab generate | occur | produced in FIG. 2 is illustrated. The direction parallel to the ruler corresponds to the rolling direction. Surface scratches extending in the rolling direction are seen in the center portion of the cut plate sample. The example in this photo is a case of very large scratches. Since a large amount of elements (such as Na) contained in the mold powder were detected by elemental analysis of the scratch occurrence point, it was specified that this surface scratch was due to the casting direction surface defect of the slab.

Fig. 3 illustrates a cross-sectional structure photograph near the surface of the austenitic stainless steel continuous casting slab in which a relatively large casting direction surface defect has occurred. The parallel direction with respect to the long side of a photograph corresponds to the width direction of the slab, and the direction perpendicular | vertical to the long side and short side of a photograph corresponds to a casting direction. Since the surface of the slab in the vicinity where the crack has been formed is dented more than the surroundings, it is considered that the distance between the solidifying shell and the mold is larger than the surroundings for some reason when the initial solidifying shell is formed. Therefore, it is thought that heat removal from the mold is gentler than the surroundings, and the solidification rate is lowered, casting proceeds while the thickness of the solidified shell is thinner than the surroundings, and stress is concentrated on the thin solidified shell portion, leading to cracking. .

In the case where this kind of cracking occurred, the metal structure near the surface of the slab was compared with the top portion near the crack, and in all cases, the dendrite secondary arm spacing was larger than the top portion in the vicinity of the crack. It was confirmed that the portion had a smaller solidification rate than the surroundings.

In order to realize uniformity and initial cooling of initial solidification, the operation (low temperature casting) which first made the difference between the molten metal temperature in a mold and the solidification start temperature of steel was examined. This was expected to reduce the mold heat removal amount as a whole. As a result of the experiment, it was possible to achieve slow cooling by low temperature casting, but it is very difficult to keep the melt temperature constant low for the entire period of the casting. When the melt temperature is too high, the effect of slow cooling is lost. On the other hand, when the molten metal temperature becomes too low, troubles, such as a blockage of the nozzle of a tundish, generate | occur | produce, and the operation | work has been impaired. Then, in addition to low temperature casting, application of the electronic stirring (EMS) in mold was examined. This is because when the electronic stirring is performed, the action of equalizing the bath surface temperature in the mold long side direction is exerted. As a result of the experiment, by the combination of both methods, initial solidification can be completely cooled and uniformized without extremely low-temperature casting, and formation of casting direction surface defects is remarkably reduced.

In addition, in the case of casting at a normal temperature without making the casting temperature low-temperature casting, even if the electronic stirring in the mold is applied, it cannot be sufficiently cooled, and the expected effect on reducing the casting direction surface defects. Was not obtained.

In the present invention, an austenitic stainless steel having the following chemical composition is intended.

As mass%, C: 0.005 to 0.150%, Si: 0.10 to 3.00%, Mn: 0.10 to 6.50%, Ni: 1.50 to 22.00%, Cr: 15.00 to 26.00%, Mo: 0 to 3.50%, Cu: 0 to 3.50%, N: 0.005 to 0.250%, Nb: 0 to 0.80%, Ti: 0 to 0.80%, V: 0 to 1.00%, Zr: 0 to 0.80%, Al: 0 to 1.500%, B: 0 to 0.010 %, Total of rare earth elements and Ca: 0 to 0.060%, remainder Fe and unavoidable impurities, wherein the A value defined by the following formula (4) is 20.0 or less.

A = 3.647 (Cr + Mo + 1.5 Si + 0.5 Nb)-2.603 (Ni + 30 C + 30 N + 0.5 Mn)-32.377. (4)

Here, the value of content of the said element represented by mass% is substituted at the place of the element symbol of Formula (4). 0 is substituted for elements that do not contain.

Although A value of Formula (4) mentioned above is used as an index which shows the ratio (vol%) of the ferrite phase in the solidification structure which arises at the time of welding, the austenite large reduction effect of the casting direction surface defect of a continuous casting slab It was also confirmed that it was a significant indicator to identify the grade of the steel. In the case of stainless steels having a value of 20.0 or less, the amount of crystallization of the δ ferrite phase during the continuous casting or the austenite single phase solidification tends to occur, so that surface defects in the casting direction tend to occur. In the present invention, such austenitic steel grades are aimed at remarkably reducing the casting direction surface defects. Steel grades in which the A value becomes a negative value may be regarded as steel grades in which austenite single-phase solidification is generally performed. Although the minimum of A value does not need to be specifically set, it is usually more effective to apply steel of -20.0 or more.

The cross-sectional structure cut | disconnected to the horizontal plane at the height of the molten metal of the molten steel in mold in the continuous casting apparatus which can be applied to this invention to FIG. 4 typically is illustrated. "Turning surface" is the liquid surface of molten steel. The mold powder layer is normally formed on the tap surface. The immersion nozzle 30 is provided in the center of the area | region enclosed by two pair of opposing molds 11A, 11B and 21A, 22B. The immersion nozzle has two discharge holes below the hot water surface, and molten steel 40 is continuously supplied into the mold from such discharge holes, and the hot water surface is formed at a predetermined height position in the mold. The contour of the wall surface inside the mold cut into the horizontal plane is rectangular, and in Fig. 4, the "long sides" constituting the long sides of the rectangle are denoted by reference numerals 12A, 12B, and the "short sides" constituting the short sides are indicated by reference numerals 22A, 22B. Doing. In addition, the horizontal direction parallel to a long side surface is called "long side direction", and the horizontal direction parallel to a short side surface is called "short side direction". In FIG. 4, the white arrow indicates the long side direction by 10 and the short side by 20. In the hot water level, the distance between the long side surface 12A and the long side surface 12B (t in FIG. 5 to be described later) is, for example, 150 to 300 mm, and the distance between the short side surface 22A and the short side surface 22B (to be described later). 5) is, for example, 600 to 2000 mm.

The back of the molds 11A and 11B are provided with electronic stirring devices 70A and 70B, respectively, and the solidification shell formed along at least the long side surface 12A and the long side surface 12B has a thickness of 5 to 10 mm. In the depth region, the molten steel can be provided with a flow force in the long side direction. Here, "depth" is the depth based on the height position of the tap surface. The hot water surface is somewhat shaken during continuous casting, but in this specification, the average hot water surface height is the position of the hot water surface. The depth region where the solidification shell becomes 5 to 10 mm thick depends on the casting speed and the heat removal rate from the mold, but generally the depth from the hot water surface is within the range of 300 mm or less. Therefore, the electronic stirring apparatuses 70A and 70B are provided in the position which can apply a flow force to the molten steel up to about 300 mm in depth from a hot water surface.

In FIG. 4, the black arrow 60A and the arrow show the molten steel direction near the long side surface generated by the electromagnetic force of the electronic stirring apparatuses 70A and 70B in the depth region where the thickness of the solidification shell becomes 5 to 10 mm. This is indicated by 60B. The flow trend by electron agitation causes long side flow in the opposite direction to each other on both long sides. In this case, in the depth region until the solidification shell thickness is about 10 mm, the horizontal flow of molten steel in contact with the solidification shell already formed becomes a swirling flow in the mold. Due to this vortex, the molten steel near the molten surface of the mold smoothly flows without causing stagnation, and the effect of equalizing the molten steel temperature in the mold when molten steel immediately below the molten surface where the initial solidification shell is formed contacts the mold wall. Increases.

FIG. 5 shows the symbols “P ₁ and P ₂ ” in the mold shown in FIG. 4 in the “long side direction 1/4 position and short side direction 1/2 position”. The average molten steel temperature T _L (℃) is expressed as the average value of the molten steel temperature (℃) of the bath surface at an average depth of 20mm in the molten steel temperature (℃) and P ₂ position of the bath surface at an average depth of 20mm in the P ₁ position.

In this invention, it casts at low temperature as possible so that following formula (1) may be satisfied. Casting to satisfy the following formula (1) 'is more effective.

10 <ΔT <50 × F _EMS +10... (One)

10 <ΔT <50 × F _EMS +8... (One)'

ΔT means the temperature difference between the molten steel temperature at the time of casting and the solidification start temperature of the molten steel. Specifically, it is defined by the following formula (2).

ΔT = T _L -T _S ... (2)

As the molten steel temperature at the time of casting, an average molten steel temperature T _L (° C) is adopted. T _L is an average value of the molten steel temperature (° C.) at an average tap surface depth of 20 mm at two places of the P ₁ and P ₂ positions shown in FIG. 5. Solidification start temperature T _S (degreeC) of molten steel can be grasped | ascertained by measuring liquidus temperature by in-lab experiment about the steel of the same composition. In actual operation, the above-mentioned ΔT can be controlled based on the data of the solidification temperature grasped for each target composition in advance.

In low temperature operation at which ΔT is 10 ° C. or lower, there is a high risk of causing trouble such as clogging of a nozzle of a tundish when an unstable temperature fluctuation occurs, and industrial practice is difficult. On the other hand, the upper limit of (DELTA) T is fluctuate | permitted according to the stirring effect of molten steel in a mold. Basically, as the stirring force by electronic stirring becomes large, the molten steel temperature near a hot water surface becomes uniform, and the permissible upper limit of (DELTA) T expands. Therefore, the effect of suppressing slab surface casting direction surface defects cannot be sufficiently obtained only by lowering ΔT without using electron stirring in the mold. However, in order to precisely evaluate the stirring effect, it was found that the influence of the discharge amount of the molten steel supplied into the mold cannot be ignored. The index which shows the stirring effect is the stirring strength index F _EMS of following formula (3).

F _EMS = V _EMS x (0.18 x V _C +0.71). (3)

Here, V _EMS is the long side average flow velocity (m / s) of the molten steel in contact with the surface of the solidification shell in the depth region where the solidification shell thickness at the center of the long side direction given by electron stirring becomes 5 to 10 mm, and Vc is the casting speed. (m / min). As the casting speed Vc increases, as the discharge flow rate from the immersion nozzle increases, stirring of the molten steel in the mold also becomes active. The stirring strength index F _EMS of the formula (3) can be understood to be a parameter corrected for the contribution of the electronic stirring to the stirring effect in consideration of the influence of the molten steel discharge amount.

By applying this stirring strength index F _EMS to said Formula (1), More preferably, (1) ', the permissible upper limit of (DELTA) T can be estimated accurately. Specifically, ΔT is smaller than 50 × F _EMS +10 as shown in formula (1), and more preferably ΔT is smaller than 50 × F _EMS +8 as shown in formula (1) '. By performing continuous casting on losing conditions, the surface flaw of the cold rolled sheet steel resulting from the casting direction surface defect can be remarkably reduced. The higher the strength (stirring strength index F _EMS ) of molten steel stirring, the wider the upper limit of ΔT becomes. However, when the F _EMS becomes excessively large, the surface of the molten metal becomes more wavy, and foreign matters such as mold powder particles and inclusions floating on the molten metal are easily attracted to the solidified shell.

In order to exhibit the effect of preventing the occurrence of surface scratches in the cold rolled steel sheet due to the casting direction surface defects at a higher level, in addition to the above formula (1) or (1) ', the following formula (5) is further satisfied. It is more preferable to control continuous casting conditions so that it is more preferable to satisfy following formula (6).

ΔT ≦ 25... (5)

ΔT ≦ 20... (6)

Moreover, in order to effectively prevent the mixing of the foreign material resulting from the wave of the hot water surface, it is more preferable to control continuous casting conditions so that following formula (7) may be satisfied, and it is still more preferable to satisfy following formula (8).

F _EMS ? (7)

F _EMS ? (8)

The metal structure photograph of the cross section perpendicular | vertical to a casting direction is illustrated about the continuous casting slab of the austenitic stainless steel which concerns on this invention obtained by the method using the electronic stirring in FIG. The direction parallel to the long side of the photograph is the width direction of the slab, and the direction parallel to the short side is the thickness direction of the slab. In this photograph, the lower end of the photograph is a field of view corresponding to a distance of 15 mm from the slab surface (mold contact surface), and the slab surface is on the upper side of the photograph.

When molten metal flows with respect to the mold, it is known that the inclination of the crystal progresses by inclining to the upstream side of the flow, and the inclination angle of the crystal growth increases as the flow velocity increases. In the example of FIG. 6, the growth direction of a dendrite primary arm inclines to the right. Therefore, it can be seen that the molten steel in contact with the solidification shell was flowing from the right side to the left side of the photograph. The relationship between the flow rate of the molten steel in contact with the solidification shell and the inclination angle of crystal growth can be seen, for example, by solidification experiments using a rotating rod-like heat eliminator. Based on the data previously obtained by in-lab experiments, it is possible to estimate the flow rate of the molten steel that the solidification shell contacts with during continuous casting. The long-term average flow velocity V _EMS of the molten steel in contact with the surface of the solidified shell in the depth region where the solidified shell is 5 to 10 mm thick is the average inclination angle of the dendrite primary arm at a distance of 5 to 10 mm from the surface by this cross section. It can be found by measuring. In the example of FIG. 6, V _EMS is estimated to be about 0.3 m / s. It is practical for a continuous casting apparatus to adjust V _EMS in the range of 0.1 to 0.6 mm / s, for example. You may manage so that it may become 0.2 to 0.4 mm / s.

In actual operation, the molten steel flow rate V _EMS can be controlled by a current value (hereinafter referred to as "electronic stirring current") applied to the electronic stirring device. In a continuous casting facility equipped with an electronic stirring device, a computer simulation, a measurement experiment of a molten steel flow rate, and a structure observation as described above with respect to the slab collected in a large number of operations records the "electromagnetic agitation current and each position in the mold. Relationship between the molten steel flow velocity at &quot; In the actual operation, the V _EMS may be controlled to a predetermined value by the electronic stirring current based on such accumulated data.

The metal structure photograph of the cross section perpendicular | vertical to a casting direction is illustrated about the continuous casting slab of the austenitic stainless steel obtained by the method which does not use the electronic stirring in FIG. The observation position of a sample is the same as that of FIG. In this case, there is no inclination in the constant direction in the growth direction of the dendrites. That is, it can be seen that the part of the cast shell having a thickness of 5 to 10 mm is solidified in the state where the long side flow of molten steel does not occur.

Example

The austenitic stainless steel of the chemical composition shown in Table 1 was cast by the continuous casting apparatus, and the slab (slab) was manufactured.

Continuous casting molds are common water-cooled copper alloy molds whose contact surfaces with the molten metal are composed of copper alloys. About the mold size of continuous casting, in the hot water level, the short side length was 200 mm and the long side length was set in the range of 700-1650 mm. The dimension at the lower end of the mold is slightly smaller than the above in consideration of solidification shrinkage. The immersion nozzle provided what has two discharge holes in the both sides of a long side direction in the center position of a long side direction and a short side direction. The outer diameter of the immersion nozzle is 105 mm. The two discharge holes are symmetrical with respect to the plane parallel to the short side surface past the nozzle center. Electromagnetic stirring apparatus was provided in the mold back surface of the opposing long side, respectively, and the electronic stirring was performed so that the flow force of a long side direction may be provided to the molten steel from the depth position near the hot water surface in a mold to a position of about 200 mm. As shown in FIG. 1, the flow direction was made into the reverse direction in the opposing long side side. Long-term average flow velocity V _EMS of molten steel in contact with the surface of the solidification shell in the depth region where the solidification shell is 5 to 10 mm in thickness is calculated in advance for this continuous casting facility. Control by adjusting the electronic stirring current based on the accumulated data. And Fig. 5 are measured by the molten steel temperature (℃) in the average bath surface depth of 20mm at two locations in the P _1, P ₂ position shown in thermocouple, employing the average value of the two locations as the average of the molten steel temperature T _L (℃) did.

In Table 2, the casting conditions of each example are shown. ΔT is the difference between the average molten steel temperature T _L (° C.) and the solidification start temperature T _S (° C.) represented by the above formula (2). The solidification start temperature T _S (° C.) is listed in Table 1. In the column of "(1) Determination", (circle) was shown when it satisfy | filled the requirements of said Formula (1), and x was not satisfied.

Each of Example No. of Table 2 produced several continuous casting slabs about 8 m in length according to the continuous casting conditions. One of them was selected as the representative slab of the example No. The one-sided surface of the representative slab was visually observed, and the presence or absence of the casting direction surface defect accompanying surface cracking was examined. When the existence of surface cracks can be clearly confirmed with the naked eye, "Slab surface cracks; Yes ”is shown in Table 2.

The representative slab of each example No. was made into the cold rolling coil of 0.6-2.0 mm of plate | board thickness in a normal hot rolling process and a cold rolling process. No care was taken with the grinder on the slab surface. The obtained cold-rolled coil was plated on the line provided with the laser irradiation type surface inspection apparatus, the one-side surface of the coil was examined by the constant detection criteria over the whole length, and the presence of the surface flaw was investigated. When surface flaw was detected in the area | region (henceforth a "segment") which divided the coil full length for every 1 m of longitudinal directions, it recognized that the segment was "a flaw segment." The ratio of the number of "scratched segments" to the total number of segments of the coil full length (hereinafter referred to as "defect rate") is obtained, and the case where the defect rate exceeds 3% is X (surface property; defect). ), The case of 3% or less was determined as ○ (surface property; good). The result is shown in the column of "cold-rolled coil surface flaw evaluation" in Table 2. This detection criterion is quite stringent, and flaws other than the flaws resulting from the casting direction surface defect of a continuous casting slab are also detected. Usually, although the said cold-rolled coil whose defect generation rate exceeds 3% is applicable to many uses, it cannot be used for the use which places importance on surface property. On the other hand, it can be evaluated that the cold rolled coil having a defect rate of 3% or less exhibits very good surface properties, and the limit on application due to scratches is extremely small.

8, the graph which plotted the relationship of (DELTA) T and F _EMS of Table 2 is shown. The o mark and x mark on the plot match the cold-rolled coil surface scratch evaluation shown in Table 2. In FIG. 8, (DELTA) T upper limit permissible boundary ((DELTA) T = 50 * F _EMS + 10) of Formula (1) is shown by the dotted line. Even if ΔT is larger than this line, there is an example in which the surface scratches of the cold rolled coil are very small and evaluated as ○. However, in order to stably realize the good surface property of (circle) evaluation, it is extremely effective to employ | adopt the conditions which (DELTA) T becomes below this line.

10 Long side direction
11A, 11B Mold
12A, 12B long side
20 short sides
21A, 21B Mold
22A, 22B Short Side
30 immersion nozzle
40 molten steel
42 solidified shell
60A, 60B molten steel direction by electronic stirring
70A, 70B Electronic Stirring Device

Claims (5)

In continuous casting of steel using a mold in which the contour shape of the mold inner surface cut into the horizontal plane is a rectangular shape, the two mold inner wall surfaces constituting the long side of the rectangle and the two mold inner wall surfaces constituting the short side are "long sides". When we call "short side", the horizontal direction parallel to the long side "long side direction", and the horizontal direction parallel to the short side is called "short side direction",
From the immersion nozzle having two discharge holes provided in the center of the long side direction and short side direction in the mold, in mass%, C: 0.005 to 0.150%, Si: 0.10 to 3.00%, Mn: 0.10 to 6.50%, Ni: 1.50 to 22.00%, Cr: 15.00 to 26.00%, Mo: 0 to 3.50%, Cu: 0 to 3.50%, N: 0.005 to 0.250%, Nb: 0 to 0.80%, Ti: 0 to 0.80%, V: 0 to 1.00 %, Zr: 0 to 0.80%, Al: 0 to 1.500%, B: 0 to 0.010%, total of rare earth elements and Ca: 0 to 0.060%, remainder Fe and inevitable impurities, consisting of the following formula (4) The molten steel of the austenitic stainless steel of the chemical composition whose A value is defined as 20.0 or less is discharged, and at least in the molten steel near the solidification shell in the depth region where the solidification shell thickness at the central position of the long side direction becomes 5 to 10 mm. On the long side, electric power was applied to generate a long side flow in the opposite direction to each other, followed by electromagnetic agitation (EMS), to satisfy the following Equation (1). A method for producing an austenitic stainless steel slab, in which continuous casting conditions are controlled.
10 <ΔT <50 × F _EMS +10... (One)
However, ΔT and F _EMS are represented by the following formulas (2) and (3), respectively.
ΔT = T _L -T _S ... (2)
F _EMS = V _EMS x (0.18 x V _C +0.71). (3)
Here, T _L is the mean molten steel temperature (° C) at an average bath surface depth of 20 mm at the 1/4 position in the long side direction and 1/2 position in the short side direction, T _S is the solidification start temperature (° C) of the molten steel, and F _EMS is The stirring strength index, V _EMS, is the long side average molten steel flow velocity (m / s) in the depth region where the solidification shell thickness at the center of the long side direction given by the electronic stirring becomes 5 to 10 mm, and V _C is the length of the casting slab in the longitudinal direction. Casting speed (m / min) corresponding to the speed.
A = 3.647 (Cr + Mo + 1.5 Si + 0.5 Nb)-2.603 (Ni + 30 C + 30 N + 0.5 Mn)-32.377. (4)
Here, the value of content of the said element represented by mass% is substituted at the place of the element symbol of Formula (4). The method for producing an austenitic stainless steel slab according to claim 1, wherein the continuous casting conditions are further controlled to satisfy the following formula (5).
ΔT ≦ 25... (5) The method for producing an austenitic stainless steel slab according to claim 1, wherein the continuous casting conditions are further controlled to satisfy the following formula (6).
ΔT ≦ 20... (6) The method for producing an austenitic stainless steel slab according to any one of claims 1 to 3, wherein the continuous casting conditions are further controlled to satisfy the following formula (7).
F _EMS ? (7) The method for producing an austenitic stainless steel slab according to any one of claims 1 to 3, wherein the continuous casting conditions are further controlled to satisfy the following formula (8).
F _EMS ? (8)

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